The present invention relates to a phase change optical recording medium capable of multilevel recording.
Optical disk memories capable of reproducing or recording and reproducing information by laser beam irradiation are widely used as mass capacity, high-speed accessible and portable storage media for data files such as audio, video and computer data and expected for further development. In order to improve storage density of such optical disks, many techniques are proposed: for example, use of a short-wavelength gas laser for master disk cutting, use of a short-wavelength semiconductor laser as an operating light source, increasing the numerical aperture of an objective lens, reducing the thickness of the disk, and so on. For recordable optical disks, mark length recording and land-groove recording, in addition to the foregoing techniques, are proposed.
Also, as a high-density oriented technique, a method of recording and reproducing multilevel data using a multilayered recording medium has been proposed. A simplest multilayered recording medium comprises two or more recording layers, which are allocated to different focal points and accessed separately for recording or reproducing. The method may provide higher reliability in both recording and reproducing operations. However, because only one of the recording layers can be accessed at once, it is difficult to perform high-speed recording and reproducing operations.
There is proposed a magnetooptical recording medium comprising two or more recording layers arranged within the depth of focus of a laser beam with an attempt at multilevel recording in respective recording layers by using various levels of laser power or recording field intensity and multilevel reproducing by analog processing of read-out signals. The read-out signals from such a magnetooptical recording medium are, however, based on slight Kerr rotation angle from which a desirable level of carrier-to-noise ratio (CNR) can only be obtained by binary digital processing. Therefore, it will hardly be feasible to subject the read-out signals to successful multilevel analog processing.
On the other hand, in a phase change optical recording medium, particularly rewritable medium, intense read-out signals can be obtained, so that it is expected to realize multilevel processing. However, such a multilayered phase change optical recording medium capable of recording and reproducing multilevel data simultaneously at a high speed has not yet been in practical use.
It is an object of the present invention to provide a multilayered phase change optical recording medium that is capable of recording and reproducing multilevel data at a high speed.
A multilevel phase change optical recording medium according to the present invention comprises first to N-th phase change optical recording layers (Nxe2x89xa72), wherein an i-th recording layer and a j-th recording layer, which are two recording layers arbitrarily selected from the first to N-th recording layers, meet the conditions of:
Ti greater than Tmi and xcfx84wi less than xcfx84xi, and
Tj less than Tmj or xcfx84wj greater than xcfx84xj,
with respect to a particular recording laser beam selected from recording laser beams having different power levels, where T is the maximum temperature of the recording layer in a recording operation, Tm is the melting point of the recording layer, Tx is the crystallizing temperature of the recording layer, xcfx84w is a time required for the recording layer to be cooled down from Tm to Tx after the laser beam irradiation, and xcfx84x is the crystallizing time of the recording layer.
A method of recording and reproducing for a multilevel phase change optical recording medium comprising two or more recording layers having different melting points and/or crystallizing temperatures according to the present invention comprises the steps of: irradiating the multilevel phase change optical recording medium with recording beams having different power levels, thereby performing recording; and irradiating the recorded multilevel phase change optical recording medium with a reproducing beam to detect reproducing signals, followed by digitizing the reproducing signals, thereby performing reproducing.
Additional objects and advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and obtained by means of the instrumentalities and combinations particularly pointed out in the appended claims.